Expansion tank or PRV?

I have some water hammer noise that seems to be from the incoming line. The water company refuses to do anything with the meter saying they have never had a meter do anything like this. My plumber says the meter is probably it. I am getting very frustrated. I would like to fix the source of the water hammer but expansion tank or PRV seems like my next best option. My pressure runs 65 - 70 PSI. I am thinking an expansion tank right as the line comes into the house might be more appropriate than a PRV. Looking for some other opinions...

Seems to me the PRV might not respond quick enough to the water hammer and if it did would make the pressure surges worse in the incoming line. I have an expansion tank at the inlet to the water heater but it is some distance from where the line comes into the house.

the meter is not causing the problem. you need to find where the hammer is taking place. usually at the end of a line or anywhere there are fast closing valves and then you need to install a hammer arrestor at that point. installed anywhere else will not make a difference.

I don't have a PRV! I am just considering putting one in. Either that or an addiditional expansion tank. But I agree with Wally that I really need to find the cause. It appears to be near the incoming line. Everyone says I need to narrow it down, check toilets, etc. but I have already done that and it isn't anything in the house, at least as far as I can determine and I have shut off everything (a few things at a time). If I shut off the main line where it enters the house I don't have the pipes banging but there may still be pressure surges in the main line - that really doesn't prove anything (at least that is my thinking). The noise happens when nothing is being turned on or off. That's why I am thinking a seal on the main line may be leaking a little (although it would have to be very little as I checked leakage at the meter too). Would a mechanics stethoscope help? I have seen some suggestions of using it on the meter.

The plumber said the problem was that my expansion tanks were water logged but after I told him I checked that and aired them up (they were low) without fixing the problem he said it was probably the meter. Seems like I need to test the main line. If I need a professional tell me exactly what I should be looking for as far as qualifications as I already got a plumber that is well respected in the area and that didn't help.

I haven't made any progress on the noise. I have eliminated things in the house. I observed that the noise happens when the line pressure sags. My theory is that the air chambers push water back to the meter and the check valve at the meter is chattering. I don't know for sure that the meter has a check valve but think it does. I may pull the meter to check.

There are no commercial companies nearby. The water company said their meter could not cause the noise and would replace the meter only if I agreed to pay for checking the meter if they didn't find a problem and said it would be $400 - $500! They send them off and have them checked so they may just be checking accuracy and not check valves or noise issues.

What size arrestor should in install where the main comes into the house? There is 500' of 2" PVC to the street. 1" comes in to the house and I have a sprinkler system which taps off where the line comes onto the house. I have expansion tanks at the water heaters but they are some distance from where the water comes in.

First, you need to understand what water hammer is, and you may not have it. When water is moving, like anything, it has inertia. IF you have a valve that can shut off quickly, that kinetic energy will try to keep the pipe moving, and that can create a noise when the pipe hits something. That is a water hammer. If the force is large enough, it can shake the whole line where the water is moving, and make nasty noises. The most common things that cause this are solinoid valves that can open and close quickly. Examples are in the dishwasher, washing machine, ice-maker, and sprinkler systems.

To effectively stop the water hammer, the arrestor must be at the end of the line where that offending valve is. you can't absorb the moving water's energy at the source (say your water meter), except if it 'bounces' back. In that case, you'd always still get one bang out of it. Severe water hammer can stress valves, hoses, and pipes and fittings.

If it is a static pressure related problem, it is not water hammer. If you have a prv, you also need an expansion tank. A leaking prv or any check valve that does not hold with a differential pressure can make noises when it (intermittently) leaks, or even if it is constant (vibrates like a reed instrument). An expansion tank resolves differential pressure buildup caused by the water heater and is required any time you have a check valve in the system (typical one is a prv, but some water suppliers put them in or around the meter). A check valve on the water supply is being installed more and more places to protect other users from problems in your home by prevently polluted water from backflowing into the system to affect others. So, people that never needed an expansion tank before, now will need one. A failed expansion tank will often cause noise and T&P valve water release.

I suppose I should be thankful for any replies but my question of how big an arrestor is still unanswered... And a technical definition of water hammer is not that helpful. What ever you want to call it - it fits your description ("If the force is large enough, it can shake the whole line where the water is moving, and make nasty noises.")

And it isn't the sprinkler system. The noise happens when the sprinkler system isn't on. Heck, it even makes noise when the manual valve to the sprinkler system is closed (not as it is being closed but when it has been closed previously and remains closed).

If it is static pressure making a valve leak I haven't been able to locate it. And the noise often happens when the pressure is dropping. If it was static pressure I would expect it to leak when the pressure was near it's peak.

I have an expansion tank but no PRV. I suspect there is a check valve in the meter and will try to confirm that next week.

Have you verified your expansion tank has not failed? they don't last forever. Even if it is not waterlogged, the pressure could be too low for it to absorb any expanding water when the WH is on. Have you installed and monitored the pressure? If you use a lot of hot water, then watch the pressure, it should NOT go up. If it does, the expansion tank is shot, incorrectly located, disconnected by a valve, or low in pre-charge.

Water hammer only occurs when water is running, then shut off. Otherwise, it is not a water hammer. If you have that, then it's likely a check valve. It could possibly be some motorized thing touching the pipes or something that is transmitting vibrations to the piping that is totally disconnected from the water supply.

I have checked my expansion tanks. They were low so I pumped them up but that did not solve the problem. I was able to pump them up with no pressure in the water lines so they seem to be ok. I have a pressure gauge installed. I have noticed that the pressure varies about 5 PSI up or down but no big variations.

It is not a motor or the like touching the pipes. Maybe a hammer "touching" the pipes? It is pressure pulses in the pipes at several times per second. The pressure gauge needle vibrates when the thumps occur but is not fast enough to show the peaks.

I have ran into a problem similar to this on several occassions. I have found that the washers on shut off valves have come lose and "rattle" during periods of water usage. My suggestion would be to check the shut off valves throughout the house before investing a lot of money on what may well be a wild goose chase.